Wednesday, March 14, 2012

Plant Neurons? Sensation and action in the Venus Flytrap

While they don't have neurons in the proper sense, they have sensory receptors, ion channels, action potentials, and can process information. One of the most remarkable feats of plant information processing occurs in the venus flytrap.

The venus fly trap is remarkable among plants because it has very fast and very specific information processing capabilities. It can sense changes in its environment and act upon its sensations quickly.

Here's how it works, illustrated by 3 fascinating studies.

1.Sensation: The venus flytrap has 3 little hair-like protrusions (see above photo) on each side of the 'mouth'. These 'trigger hairs' contain mechanosensory cells which activate when the hair is moved.

In order to see what these cells are actually like, Benolken and Jacobson used electrophysiology methods (similar to the ones used in animal neurons) to record the cells' electrical signals during mechanical stimulation.

up close image of a trigger hair.
The sensory cells are at the indentation
point near the bottom. (source)

They found that the primary sensory cells were right near the base of the hair where there is a distinct indentation. In other words right where the trigger hair would bend if a delicious fly bumped into it.

They also measured the force needed to stimulate these cells. Interestingly in more mature plants, the trigger hairs are stiffer and require more force to activate the mechanosensory cells. (Why this would be, I am not sure. Maybe larger prey hits the trigger hairs harder, and the larger older traps don't want to bother with puny meals.)

When enough of these sensory cells are activated, they trigger the second step in the process, the action potential.

2. Information travels from the base of the trap through the two sides of the 'mouth' and makes it close.

Here is where it gets tricky. I couldn't find any detailed information on how the sensory cells are connected to the base of the trap (also called the midrib). These sensory cells must 'project' to the midrib, but how these projections are shaped, where they converge, and how they relay their signal to other cells is a mystery to me. (If you know or have a reference please post it, I am curious about this missing step.)

However, once the information gets to the base of the trap, it takes on the form of an action potential which travels through each side of the leaf. To investigate this action potential, Volkov et al. (2007) placed silver-chloride wires directly into the midrib and lobes of a trap and record the electrical transients. They find that when they stimulate the hairs, an action potential can be recorded through these wires. They also find that by putting current directly through the wires, they can cause the trap to close even without stimulating the hairs.

They are able to inhibit this action potential by applying ion channel blockers to the soil of the plant 2+ days before the experiment. The application of TEACl (a potassium channel blocker) prevented the trap from closing when the hairs were triggered and when the electrodes were directly stimulated. The application of calcium channel blockers caused the trap closure to be much slower. So the information traveling step requires potassium and is pretty reliant on calcium too. So once the action potential is triggered, how does that actually make the trap close?

3. Trap Closure
There are several theories about how the trap actually closes. Some involve actual cell growth, but Foterre et al., (2005) show that the trap can close based mainly on mechanical principles.

To visualize the specifics of trap closure, Foterre et al. paint uv-sensitive dots in a grid on each trap lobe. This technique is surprisingly like what this person did using white-out to test their own garden of flytraps. Basically Foterre et al. show that there is a biophysical trigger that changes the curvature of one part of the plant, but that as soon as that curvature is changed the rest of the process is a passive response based on mechanical principles. That is, the 'snapping' closed has a passive component that can be modeled computationally as a thin elastic sheet. They summarize it nicely in their last paragraph:

"Upon stimulation, the plant 'actively' changes one of its principal natural curvatures, xn, the microscopic mechanism for which remains poorly
understood. Once this change occurs, the geometry of the doubly-curved
leaf provides the mechanism by which elastic energy is both stored and
released, and the hydrated nature of the leaf induces the rapid damping
that is equally crucial for efficient prey capture. A single geometrical
parameter () determines the nature of closure: if c 0.8, the leaf closes smoothly, and if > c, the leaf snaps rapidly. This ingenious solution to the
problem of scaling up movements and speed from the cellular to the organ
level in plants, nature's consummate hydraulic engineers, shows how
controlling elastic instabilities in geometrically slender objects
provides an alternative to the more common muscle-powered movements in
animals."

So there you have it, everything you ever wanted to know about the Venus Flytrap and then some. Even though the flytrap electrical properties have been studied for hundreds of years, there is so much that is not known. I dare say we know more about the neurons of the mouse brain than we know about the sensory cells in the Venus Flytrap.